A model is proposed in which the conduction band is discretised into energy intervals corresponding to longitudinal-mode separations for the simulation of inhomogeneous broadening and mode beating effects in semiconductor lasers. Four first-order relaxation processes are proposed to take account of intraband relaxations, and mode beating is included as a separate nonlinear gain term in the rate equations. In addition to the usual results of spectral-hole burning, single longitudinal-mode stabilisation and increased damping of relaxation oscillations, the model also accounts for the excitation of nonadjacent longitudinal modes experimentally observed at high bias in narrow planar-stripe lasers and the temperature dependence of spectral behaviour in CSP lasers.